Mueller's trip comes as the German automaker and U.S. regulators are at an apparent impasse over how to proceed with the expected recall of nearly 600,000 "clean diesel" vehicles sold with secret software designed to make their engines pass federal emissions standards while undergoing laboratory testing. The vehicles then switch off those measures in real-world driving conditions, spewing harmful gases at up to 40 times what is allowed under federal environmental standards.

"We've been having a large amount of technical discussions back and forth with Volkswagen," McCarthy said at a public event on Thursday. "At this point, we haven't identified a satisfactory way forward, but those discussions are going to continue. We are really anxious to find a way for that company to get into compliance, and we're not there yet."

On Monday, the Justice Department, representing EPA, filed a civil suit that could potentially expose VW to more than $20 billion in fines under the Clean Air Act. VW could rack up additional civil penalties based on facts determined at trial.

VW first admitted in September that the suspect software was installed in cars with its popular 2.0-liter diesel engines. The company has thus far denied findings by U.S. regulators that another so-called "defeat device" was also included in a smaller number of diesel vehicles with 3.0-liter engines, including some sold under the VW-owned Audi and Porsche brands.

Bringing the cars into compliance with U.S. clean-air rules will likely include complicated recalls to install either new software or additional equipment on the cars. The process is expected to take several years.

Mueller has apologized for the scandal, but denies he and other corporate leaders at the company knew of the scheme. He has suggested a small number of software developers in Germany are to blame for the suspect computer code.

Worldwide, the company says cheating software was included in more than 11 million vehicles. The company has hired a U.S.-based law firm to conduct an internal investigation. The findings of that review have not yet been made public.

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AP auto writer Tom Krisher reported from Las Vegas. Follow him at https://twitter.com/tkrisher . His work can be found at http://bigstory.ap.org/content/tom-krisher

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Follow AP environmental writer Michael Biesecker on Twitter at https://twitter.com/mbieseck and find his work at http://bigstory.ap.org/content/michael-biesecker